The present invention relates to a method and system for managing power flow in an electric power network or grid. More specifically, the invention relates to the implementation of switching apparatus for a segment of a bundle conductor electric power line in a network or grid, with the switching apparatus being distributed throughout the grid and controlled in a manner to manage power flow. In the following text, we will refer to xe2x80x9cphase linexe2x80x9d to describe what is usually known by a person skilled in the art as xe2x80x9cphasexe2x80x9d. The system and method can thus be used for modifying, in a static or dynamic manner, the power flow through an electric power line.
Power flow in an electric power transmission grid is difficult to manage. Power is transmitted over transmission lines from power sources, i.e. generating plants, to loads over great distances. In a typical network, power flows over multiple paths to any given load. Each transmission line has a maximum flow capacity at which: 1) an accident causing loss of the line would not disrupt the grid stability; and 2) the thermal limit of network components, such as transformer and transmission line components, are respected. Maintaining power flow within safe and stable limits on each transmission line while meeting the demands at loads at a variety of geographically distinct locations is an art. Power will flow in a network along the paths of least impedance, and typically, as there is an increase in demand at a point in the network or a change in network topology due to a loss of a transmission line, the natural change in power flow in the transmission lines of the network does not favor stability or operating within safe thermal limits. In response to such a change in the network, administrators of the network typically control power flow by adjusting, where possible, loads or power injected by generators, and, during extreme conditions, the removal or insertion of transmission lines, all according to the geography and topology of the network. To ensure safe operation and/or stability of the network as a result of a sudden change, the maximum flow capacity of transmission lines are set within very conservative, albeit necessary, values. This reduces the practical transmission capacity of the network and requires installation of more transmission lines.
In more recent years, the challenges of power flow management in electrical transmission grids in the United States have significantly increased due to integration of power systems across ever increasing regions of the US. Changes in demand for power at different points in the grid can cause very complex problems. These problems can result in grid congestion, the consequences of which include price spikes, load dropping, and if these measures do not suffice, loss of power distribution to one or more areas. As mentioned above, changes in demand and/or source, or the loss of a transmission line generally do not favor network stability. Managing such changes by source/load adjustment is awkward. To improve power flow management, devices known as flexible AC transmission systems (FACTS) have been proposed. These devices include phase-shifting transformers, impedance modulators, series compensation capacitors, and the like, that are installed at transmission line stations to adjust power flow in each transmission line, so that power can be guided to flow in a safe, stable and balanced manner in the large number of lines within the network.
FACTS are usually referenced electrically with respect to ground. Due to the transmission line voltages in the range of 100 to 800 kV, the cost of such devices is very high. FACTS require the addition of new components for providing the power flow control without exploiting the properties of existing network components.
Unfortunately, FACTS are not widely implemented in power distribution networks in the US, in such a way that would allow power flow management to be made easier. The small amount of power being adjusted in each transmission line is nevertheless a significant quantity of power to be handled by a device inserted into a transmission line. Due to the power involved and the fact that devices are ground referenced, the cost of such FACTS is not negligible.
It is a first object of the present invention to provide a method and system for power flow management in an electric power network or grid that use devices to modulate the impedance or to implement any other FACT-related function of phase lines in bundled conductor transmission line segments in the network in a manner which exploits the characteristics of bundle conductors and operates on the transmission line with no ground reference. The invention may thus provide a low-cost FACTS.
It is a second object to provide a method and system for power flow management in an electric power network or grid, in which a number of phase line impedance modulating devices are distributed over the transmission line at various segments.
It is a third object to provide a method and system for power flow management in an electric power network or grid, in which a number of bundle conductor impedance modulating (or other FACT-related function) devices are remotely controlled by telecommunications.
It is a fourth object to provide a method and system for power flow management in an electric power network or grid, in which a number of impedance modulating devices are provided in a number of transmission lines within the network.
It is a fifth object to provide a method and system for power flow management in an electric power network or grid, in which a number of phase line impedance modulating devices modulate either real or reactive power flow, or both, in the phase lines.
It is a sixth object to provide a method and system for power flow management in an electric power network or grid, in which a number of transmission line impedance modulating devices operate without having an electrical reference to the ground.
It is a seventh object to provide a method and system for power flow management in an electric power network or grid, in which a number of phase line impedance modulating devices modulate only the impedance of some but not all phase line bundle conductors, and electronic switches are used for switching modulated phase line conductors.
It is an eighth object to provide a method and system for power flow management in an electric power network or grid, in which a number of phase line impedance modulating devices are mounted on an insulated portion of at least one transmission tower of the transmission line. In lines requiring greater impedance control, a number of devices may be installed at a number of towers along the transmission line. Advantageously, impedance modulating circuit elements, such as resistors or capacitors may be mounted within the volume defined by four phase line conductors of the phase lines.
It is a ninth object to provide a method and system for de-icing bundle conductor power transmission lines. Preferably, when in the process of de-icing, the system is used to concentrate power flow in transmission lines where de-icing is underway.
It is a tenth object of the invention to provide a method and system for power flow management in an electric power network or grid, in which at least one line impedance modulating device is mounted on an insulated portion of at least one transmission tower of the transmission line. In the case of polyphase lines, the device can adjust impedance between phases.
According to a broad aspect of the invention, there is provided a method of power flow management in an electrical power distribution network, the method comprising steps of: providing at least one switching unit mounted to insulated portions of transmission line towers in proximity to at least one bundle conductor for opening and closing a current path in at least one of a plurality of conductors of the bundle to cause an impedance change modulating power flowing in at least one conductor of the at least one bundle conductor in an electric power transmission line at one or more segments in the network; providing insulation between the conductors of the bundle so as to be able to withstand a voltage difference between the conductors caused by the switching unit in the segments; and managing the power flow in the segments by changing the series impedance of the line by controlling the at least one switching unit.
While some conventional bundle conductor spacers have a rubber sleeve between the spacers and the conductors, which sleeve can provide adequate insulation for up to a few hundred volts, it is preferred that conventional spacers are replaced by insulating spacers. With better insulating spacers, spans may reach 20 to 50 km. Most yokes are not insulating by design. As with spacers, yokes must be insulating in the present invention, therefore, yokes are replaced by insulating yokes.
While most segments will span about 10 to 50 km, the present invention can be applied to provide a current limiting FACTS, in which case a shorter span of a few kilometers is sufficient. A current limiter can also detect when a current threshold is exceeded using a local detector and provide local automatic of the switching unit.
The step of managing may comprise analyzing power flow in a plurality of electric power lines of the network, determining a desired change in impedance for at least one of the plurality of electric power lines of the network, and controlling at least some of the switching units to implement the desired change in impedance.
The step of controlling may comprises transmitting a control signal to the switching units via a communications network, which may be wired or wireless.
The control signal may comprise a plurality of switching units commands addressed to specific ones of the switching units, the step of determining comprising selecting a combination of switching units and impedance change values expected to result in the desired change.
The series impedance change may be a variable increase in impedance caused by opening one or more of the at least one phase line conductor, the combination of switching units and impedance change values being selected to reduce excess joule heating at points in the phase line.
The combination of switching units and impedance change values may be changed periodically to yield the same desired impedance change while allowing phase line conductors to cool.
The series impedance change may be a variable increase in impedance caused by opening one or more of the at least one phase line conductor. One phase line conductor may permanently left unswitched so as to conduct, or the switching units may comprise electromechanical switching units having a plurality of switching states that prevent an open circuit as the switching states are changes.
The switching units are preferably provided at an insulated portion of transmission towers in two groups for modulating impedance of two successive segments from each tower.
The switching units may be provided at transmission towers in two groups for modulating impedance of two the segments from each tower, the two groups of switching units sharing a same communications interface. The control signal may be transmitted to communications interfaces at the towers, the communications interfaces communicating in turn with the switching units via a local wireless communications interface.